Chromosome abnormalities are a common finding in many human malignancies. In childhood leukemia, translocations are the single most important prognostic variable and their presence in leukemic blast cells at diagnosis correlates with high risk disease and a poor prognosis. Translocations involving human chromosome 11q23, a known fragile site, are present in both childhood and adult acute leukemias. The t(4;11) translocation is associated with acute undifferentiated leukemia. Using the approach that has been successfully applied to mapping the neurofibromatosis translocation breakpoint (P. O'Connell et al.), a translocation breakpoint on human chromosome 11q will be generated by micro cell fusion for mapping of the 4q21 and 11q23 regions. These hybrids will contain fragments of either chromosome 4q or 11q in a rodent parent cell. The panels of marker DNA probes which have been mapped to chromosome 4q and 11q by linkage analysis will be physically mapped to the breakpoint regions. Additional probes for mapping will be generated from cosmid clones derived from the breakpoint region. Using pulsed field gel electrophoresis, probes which are known to physically map on either side of the breakpoint will be used to identify large restriction fragments which span the translocation breakpoint. A cosmid library will then be screened with the appropriate probes to identify the breakpoint and clone the genes located near the junction. The genes and their products will be identified and studied in normal and tumor cells to elucidate their role in the pathogenesis of leukemia Characterization of the genes present at the breakpoints and the gene products expressed in normal and tumor cells will provide a greater understanding of the molecular events leading to leukemogenesis and may possibly lead to improved therapies. The approaches described in this proposal can be applied to the isolation and characterization of any non- random chromosome translocation present in human cancer.